To understand possible changes in Earth Rotation Parameters that may be excited by the atmosphere in the current century, coupled atmosphere-ocean climate models can be used to simulate the response to a changing amounts of greenhouse gases (GHG) and other climate forcings. We turn to several models participating in versions of the Coupled Model Intercomparison Project to forecast possible fields of the winds and surface pressures needed to calculate the angular momentum.

For the axial component of angular momentum, related to expected changes in length of day, we examine variations and trends in zonal AAM by latitude band and height from several models. According to the model results, strong increases in zonal winds can occur in the upper troposphere/lower stratosphere subtropics in both northern and southern hemispheres, strengthening the wind jets there, and hence the overall zonal angular momentum. This increase tends to occur more in scenarios of the climate models with the strongest changes in GHG concentrations. This change has occurred both over the 20th century according to models and atmospheric reanalyses, and is forecast to occur over the 21st century as well, according to the simulations. We note too the surface pressures fields from which the mass component is estimated. These form important contribution to the components of angular momentum about the equatorial axes, and hence relate to possible changes in polar motion excitation for the upcoming century.